The present invention relates to a method for producing biaxially oriented polypropylene films including at least one layer, and use thereof.
Biaxially oriented polypropylene films (BOPP films) are used in a wide range of packaging applications because of their good performance characteristics. These good performance characteristics are for example high mechanical strengths, good dimensional stability and visual brilliance. Besides their use as packaging films, considerable quantities of BOPP films are also used in technical applications. These include metallisation and transfer metallisation, lamination and use an electrical insulator in capacitor manufacturing.
Various methods for producing biaxially oriented polypropylene films are known from the prior art. In the “tenter process”, the BOPP film is produced by extruding, shaping in a slot die and stretching in the longitudinal and transverse directions.
In detail, this method is carried out in such manner that the propylene polymers are compacted, heated and melted in an extruder, then the molten masses corresponding to the respective plies of the film are extruded through a flat sheet die, and the film obtained thereby is drawn on one or more rollers to stabilise it, the film is oriented and then thermofixed. Finally, the machine roll produced in this way is processed to create the cut roll ready for use by the customer.
In this process for producing biaxially oriented films, a large amount of film trimmings is created during the process itself, for example due to the edge strip. Edge strips are the borders of the film that are gripped by the clips of the lateral stretching frame but not stretched as well during transverse stretching. The unstretched border is significantly thicker than the film after orientation and must therefore be cut off. Depending on the film type and machine width, the amount of material lost with the edge strip may be as much as 25% by weight. For reasons of economy, the edge strip must be regranulated and returned to the original feedstock of raw material together with the rest of the film trimmings that is created when the cut roll is prepared, in the event of tearing or at the infeed to or outfeed from the machine.
In some application areas requiring particularly high quality standards of the film, it is not possible to reuse film trimming with the original raw material feedstock in this way again. This is true for example in the production of electroinsulating films that are used for manufacturing capacitors. For these applications, particular film properties such as low dielectric loss factor, high electrical pass resistance, high DC and AC proof voltages, and the smallest possible number of flaws are required. One of the ways to obtain these properties is to use exceptionally pure polypropylene with low ash and chlorine content. In addition, the polymers must not contain any ionogenic components. Of course, raw materials that meet these purity requirements are significantly more expensive than conventional raw materials for packaging.
It has been found that the high quality requirements for such electroinsulating films are no longer met if film trimmings is added to the ultra-pure raw material. The continuous use of trimmed film in the production inevitably results in a certain content in the film of material that has been recycled multiple times, since film material that already contains some recycled material is itself constantly returned to the recycling circuit. Material that has already been recycled several times is also decomposed and contaminated repeatedly in each new cycle, so the quality of the repeatedly recycled material becomes steadily poorer. The more cycles the respective fraction of recycled material undergoes, the lower its proportion in the finished film becomes, but the quality of this proportion is steadily degraded at the same rate.
The result of this in practice is that the film trimmings generated when manufacturing electroinsulating films becomes waste by definition, and can only be used in applications with less stringent requirements, such as production of packaging materials or injection moulding. As a result, the financial losses associated with film trimmings in the production of electrofilms are particularly significant.
Even in the field of packaging films that are produced from relatively less expensive raw materials, the quality of the film can be impaired by material that has been recycled multiple times, particularly if the fraction thereof becomes too high. In this case, therefore the proportion of material that has been recycled multiple times in the film must be monitored, and reduced if necessary. Thus, in this area too, there is a need for a method for improving economy and quality in the film manufacturing processes.
European patent EP 0 740 993 describes a method for producing biaxially oriented polypropylene films in which the border area is created separately from a lower-quality raw material. The highly pure propylene polymer of the film is remelted in a separate extruder and extruded together with this second, lower-quality propylene polymer from a second extruder. The molten mass of the second propylene polymer is directed along both sides of the first propylene polymer mass in such manner that the two molten masses are extruded together and simultaneously through the flat sheet die, and the lower-quality propylene polymer forms the border area of the film during the manufacturing process. After the film has been stretched longitudinally and laterally, this border area may be separated and used again.
In practice, it is been found that a number of disadvantages are associated with this method. The two different polymers mingle with one another at the boundary between the film border and the film in such manner that is it difficult to separate the lower-quality polymer. Either the film itself is contaminated by the lower quality polymer in the border area, so that the quality is not consistent over the entire width of the film, or an excessively wide border must be cut off, thereby reducing the yield of the method. It has also be found that after several cycles the polymer in the border area is degraded to such an extent that is must be replace with fresh material. This also renders the method less financially advantageous.
Accordingly, there is still a need for suggesting a method for producing films from high-quality, for example particularly pure, polypropylene, that avoids the disadvantages described in the preceding. The method is intended particularly to reduce the financial losses associated with film trimmings that cannot be reused.